Anomalous propagation effects can cause a large number of unwanted returns in radar systems subject to certain environmental conditions.
For the purposes of this application, the term anomalous propagation covers the different electromagnetic wave propagations not encountered in a standard atmosphere that refer to cases when a signal propagates below the normal radio horizon.
For example, as shown in FIG. 1, when a radar system 1 is located in a hot and humid environment, this environmental condition causes radar pulses to be transmitted over very long distances and below the horizon. These pulses then reflect from objects located far from the radar system, over the horizon, such as oil rigs or mountain ranges which were never anticipated to be part of the radar return.
In typical atmospheric conditions, one can normally assume that an electromagnetic wave 2 moves through the troposphere in air that decreases in temperature in a predictable way as height increases as illustrated in FIG. 1. If this is not the case, then the electromagnetic wave will follow a different path, which can lead to super-refraction or sub-refraction.
In certain situations, it can be the case that a layer of air can be cooler than the air above it, breaking the above assumptions for typical atmospheric condition. This situation is sometimes termed a “temperature inversion”, and an example of this situation is where a first layer of air near the ground starts cooling at night while another layer of air remains warm away from the ground and above the first layer.
When such a “temperature inversion” occurs, the refractive index of the air increases and an electromagnetic wave passing through the affected area is subject to anomalous propagation where the wave path bends towards the Earth's surface rather than continuing up into the troposphere as illustrated in FIG. 2.
Where the “temperature inversion” is located at the surface of the Earth, the electromagnetic beam will eventually hit the surface and a portion will reflect and be received by the radar system and the remainder will continue in the forward direction, be refracted downwards again, and hit the earth's surface again at a longer range. This may continue many times. Alternatively, where the “temperature inversion” is away from the Earth's surface, for instance in a zone where a cooler and a warmer mass of air collide, the electromagnetic beam can have its path bent within the layer of air such that it extends the distance the beam travels, possibly beyond the expected transmission distance.
The extreme of this situation is when the “temperature inversion” is very strong and shallow, such that the electromagnetic beam is trapped within the “temperature inversion” layer and the beam stays within the layer as it would behave in a waveguide. This is usually termed “ducting”. This is illustrated in FIG. 3.
In surface-based “ducting”, that is to say where an electromagnetic beam is trapped in a “temperature inversion” layer near the surface of the Earth, the beam will repeatedly reflect from the ground and then from the “temperature inversion” layer. This will cause return echoes every time the beam reflects from the ground.
The net effect of any of these anomalous propagation conditions on the performance of the Radar is that signals received at the antenna, which could normally be assumed to be returns or reflections from an object at a certain range, could actually be reflections from an object positioned significantly further away and possibly even below the usual Radar horizon. Such returns are termed anomalous targets or clutter, are range ambiguous, and can interfere with the normal processing of received signal, meaning that potential targets of interest can be lost amongst the anomalous signals. This can have an adverse effect on the performance of the Radar system and can potentially place it in danger in the event that one of the missed targets is actually a threat.
In a marine setting, where the Radar is installed on a ship, examples of the kind of objects which could cause such returns include land masses or shorelines, oil rigs, aircraft or large slow-moving vessels, such as tankers.
It is known to use so-called guard pulses, which are transmitted from the Radar ahead of the normal pulses which are to be processed by the Radar. Guard pulses are additional pulses inserted at the start of a burst, and are intended to illuminate clutter that is beyond the Radar's non-ambiguous maximum range, so that it can be cancelled by Moving Target Detection (MTD) processing in the later PRIs of the burst. Any returns received in the PRIs immediately following their transmission are not processed, but discarded.
As an example, if anomalous clutter is present in the fifth and subsequent receive periods of a burst, then 4 guard pulses can be transmitted ahead of the normal pulses. Any signals received in the first 4 receive periods, corresponding to the guard pulses are effectively ignored and only the subsequent pulses are processed and treated as valid signals. The returns from the anomalous clutter are processed by the receiver using coherent filter processing, which is able to ensure that such returns are effectively discounted.
Even though the use of guard pulses is effective in dealing with anomalous clutter, the transmission of extra guard pulses wastes valuable Radar time which, in the case of a Multi-function Radar (MFR), could be better used performing other tasks. In this way, the overall performance of the radar system can be adversely affected by transmitting guard pulses unless deemed absolutely necessary.
It is an aim of embodiments of the present invention to provide a means for identifying anomalous propagation conditions in a Radar system.
According to a first aspect of the present invention, there is provided a method of detecting an anomalous propagation condition in a Radar system, comprising the steps of: subtracting returns received in a first receive period from returns received in a succeeding second receive period, and repeating this step for a plurality of receive periods; and if the step of subtracting gives a result in excess of a predetermined threshold in one of the plurality of receive periods, then registering this as a possible anomalous propagation condition.
Preferably, an azimuthal scan is divided into a plurality of sectors, and in order to confirm an anomalous propagation condition, a possible anomalous propagation condition must be registered in at least a predetermined number of the plurality of sectors.
Preferably, the predetermined number of sectors is one.
Preferably, an operator is alerted to an anomalous propagation condition and is then able to manually alter the operation of the Radar accordingly.
Preferably, detection of an anomalous propagation condition automatically triggers the insertion of one or more additional guard pulses.
Preferably, if an anomalous propagation condition is no longer detected, the Radar automatically reverts to its previous mode of operation and any additional guard pulses are removed.
According to a second aspect of the present invention, there is provided a Radar system arranged to detect the presence of anomalous propagation conditions, comprising: a transmitter; a transmit controller, operable to supply the transmitter with a waveform for transmission; a receiver, operable to receive signals returned in response to the transmission of a waveform; and an anomalous propagation detector, operable to detect the presence of anomalous propagation conditions by subtracting returns received in a first receive period from returns received in a succeeding second receive period, and repeating this step for a plurality of receive periods; and if the step of subtracting gives a result in excess of a predetermined threshold in one of the plurality of receive periods, then registering this as a possible anomalous propagation condition and, instructing the transmit controller to insert one or more additional guard pulses into the transmit waveform.